The present application relates to thermal management of battery packs, and more particularly but not exclusively relates to battery packs in hybrid power systems. Battery packs for hybrid powered applications lose efficiency and operating life if the temperature of the battery pack is too high or too low. At early run-times for a system, the battery pack temperature tends to be low. After the application warms up and especially at high power conditions, the battery pack can generate excessive heat. Accordingly, there is a demand for further improvements in this area of technology.